Fill level meters which operate according to the pulse-runtime method, which scan a reflected echo signal, generate a set of information for each echo in the fill level envelope curve out of the scanned echo signal. The act of generating an envelope curve from received echo signals is sufficiently known so that this technique requires no further explanation (see for example DE 44 07 369 C2; M. Skolnik, “Introduction to Radar Systems”, 2nd edition, 1980, McGraw-Hill; title: Peter Devine, “Radar level measurement—the users guide”, VEGA Controlls Ltd., 2000, ISBN 0-9538920-0-X).
In fill level sensors operating according to the pulse-runtime method, short electromagnetic or acoustic pulses are emitted in a direction of a top surface of bulk material. Subsequently, the sensor records the echo signals reflected by the bulk material and by fixtures built into the container, and, from this, derives the respective fill level, taking into account the pulse propagation speed.
In this method the fill level is determined from a particular echo which can be detected in the fill level envelope curve as being representative of the fill level. In this arrangement the fill level envelope curve is scanned by an analog-digital converter, as a result of which the received fill level envelope curve is provided to a microprocessor or microcontroller in digital form for further processing. However, the received fill level envelope curve not only comprises the fill level echo which is representative for the present fill level and which, in the present document, is referred to as the useful echo, but furthermore it often also comprises parasitic echoes, which are for example caused by multiple reflections or by reflections from fixtures built into containers.
In order to only detect the real fill level echo as such in such a fill level envelope curve, or rather to filter out any undesirable disturbance reflections, often pre-processing of the fill level envelope curve becomes necessary. In this pre-processing of the fill level envelope curve the echoes are processed using one-dimensional or multi-dimensional signal processing methods, for example image processing methods such as filtering, averaging, selection and classification. The fill level envelope curve prepared in this way is then examined and analysed for echoes that are representative of the bulk material, or rather is examined and analysed for disturbance reflections, and is analysed. By means of the echoes that have been prepared in this way and that comprise, for example, data relating to location, amplitude and width of the echoes, a decision can then be made as to which echo is representative of the true fill level and which echo is not. If an echo is detected as being representative of the present fill level, then the location of the analysed echo corresponds to the sought fill level value.
Since, as has already been explained, any fill level envelope curve received always also can comprise parasitic echoes, these parasitic echoes have to be safely detected to prevent mistakenly determining the fill level from such an echo. A known criterion to assess whether any echo is a fill level echo or a parasitic echo consists of always using the echo with the highest amplitude as the fill level echo. However, this criterion has to be assessed as being relatively unsafe, since, for example, a source of disturbance in the signal propagation path, which source is located closer to the receiver of the fill level meter than is the actual fill level, as a rule will return a stronger echo than does the fill level itself. This criterion should therefore not be used on its own but always in combination with other conditions.
From DE 42 23 346 A1 an arrangement and a method for non-contacting distance measuring using pulse echo signals is known. This arrangement compares a pulse echo signal for more precise determination of the signal run times with signal samples stored in a neural network. The aim is in particular to precisely determine the signal run time even in those cases where strong parasitic echo signals are superimposed on the pulse echo signal. Using parallel data processing techniques and associative comparison of the received signals with learned samples that are stored in the neural network, it is possible to regenerate the hidden information and thus determine correct fill level data. With the use of neural associative signal processing, complex holistic evaluation of the pulse echo profile is possible. In this technique the measuring distance itself can be used as an intrinsic reference element in that compensation values are derived from existing parasitic echoes.
Furthermore, from DE 42 34 300 A1 a fill level metering method for direct determination of the useful echo without the use of a parasitic echo storage device is known, in which method the temporal shift of the useful echo, which temporal shift is caused by the changing signal run time occurring during filling or emptying of a container, is recorded, and this criterion is evaluated so as to make it possible to differentiate between the useful echo and parasitic echoes. In other words, in this method, for the purpose of differentiating between a useful echo and a parasitic echo, a check is made whether successive signal courses contain echo pulses which continuously shift in time. That continuously shifting echo that is located closest is then identified as the useful echo. The underlying idea of the known method resulting from this is that, with reflection from the internal wall of the container, signal run times are stable over time so that the position of such noise pulses is unchanged, even in repeated readings, within the receive order. However, normally this temporal positional stability within the receive profile also applies to the useful echo reflected directly from the top surface of the bulk material.
Other methods of fill level detection use echo ratios received in the past, and compare them individually with the echoes of the presently received fill level envelope curve. In these methods the received echoes of an already received fill level envelope curve are archived in a storage device so that these echoes can subsequently be compared individually with the data from a subsequent fill level envelope curve. For example, from EP 0 689 679 B1 a method is known which relates, after forming a difference value, the presently received echoes to the echoes already received in the past, and from it, using a fuzzy evaluation unit, calculates a probability at which the echo is a fill level echo. This approach is associated with problems not only because this method is only suitable for filtering out multiple echoes, but also because the method disclosed in EP 0 689 679 B1 only makes it possible to compare echoes at two points in time.
Moreover, for example, from DE 33 37 690 a method is known in which, during a teach-in phase, the positions of parasitic echoes can be archived manually in the storage of the sensor. Setting up this parasitic echo storage device can take place by measuring the empty fill level container or by manual entry of discrete parasitic echo positions by the user. After completion of the teach-in phase, the echoes received in a curve are compared with the entries in the parasitic echo storage device. Subsequently, the sensor software no longer regards known parasitic echoes as possible useful echoes.
Furthermore, from U.S. Pat. No. 5,157,639 a method is known which, according to a classification of the existing echoes into useful echoes and parasitic echoes, archives in the storage device of the sensor the information determined in relation to the parasitic echoes. In the procedure described therein, either the nearest echo or the echo with the highest amplitude is declared to be the useful echo. All other echoes detected in the echo curve are thus considered to be parasitic echoes and are archived in the parasitic echo storage device.
The above-mentioned methods share a common factor in that they have certain weaknesses in relation to their practical applicability. For example, the classical parasitic echo storage described in DE 33 37 690 requires a teach-in cycle initiated by the user. In practical application the user is thus forced to manually enter the information relating to the parasitic echoes returned by the container, or at least is forced to initiate a self-teach cycle when the container is empty.
In contrast to this, the method for automatic parasitic echo storage presented in U.S. Pat. No. 5,157,639 is not in a position to perform a reliable classification into useful echoes and parasitic echoes. From practical application, configurations are known in which a relatively large source of disturbance is present directly in front of the aerial of the sensor, whereby the described method would already reach its performance limits. Moreover, the method presented provides no information as to when which criterion is to be used when classifying existing echoes into useful echoes and parasitic echoes.
There is a further problem in that when comparing newly received echoes with the echoes of fill level envelope curves already stored, often allocation problems arise because the presently received echoes can change over time although they are always caused by the same reflection position in the container. Such problems arise for example from dust formation during filling, or from the bulk material subsequently sliding down during the process of emptying bulk containers.
However, in order to ensure safe fill level metering it is necessary for a fill level that has been recognised to be time and again recognised anew on the basis of the presently received echoes, rather than a noise reflection mistakenly being assessed as being representative of the fill level. For example, if a fill level echo temporarily cannot be acquired, this must be detected so as to prevent any allocation in which for example a parasitic echo is identified as the fill level echo. This frequently problematic allocation of echoes from past fill level envelope curves to data of a present fill level envelope curve usually takes place in that data of present echoes are compared with data of already received echoes. If in such a comparison, for example using a threshold value curve or a maximum search, a present echo corresponds to an already received echo, it is assumed that these echoes correspond to each other, as a result of which the new echo is identified as the useful echo. If the number of echoes received in an already received fill level envelope curve is different from the number of present echoes, then there is the danger of allocation errors occurring. There is also the danger of misallocation if several echoes occur in a narrow range.
Feedback of the parasitic echo storage to signal processing itself constitutes a basic problem in fully automatic parasitic echo storage. If a sensor were erroneously archive the present fill level as a parasitic echo, it would no longer be possible for the subsequently running classification algorithms to identify the correct fill level.
In the use of static parasitic echo storage, problems are always encountered if parasitic echoes appear anew, for example as a result of the bulk material caking to the container walls, or if previously known parasitic echoes disappear, for example by caking falling off. Hitherto known methods for echo evaluation with the use of a parasitic echo storage device are not capable of supplementing newly created parasitic echoes, nor are they capable of removing disappeared parasitic echoes from the parasitic echo storage device.
In the case of static fill level ratios, those methods which do not use a parasitic echo storage device (for example the method described in DE 42 34 300 A1) in order to carry out a classification into useful echoes and parasitic echoes in conjunction with direct analysis of echo movements are unable to classify existing echoes into useful echoes and parasitic echoes. The advantages of such algorithms are thus limited to periods of time during which the container to be measured is being filled or emptied.